This invention concerns a method and apparatus for the production of a suspension of fibrous material made from waste-paper including the steps of: dissolving; cleaning; stain removal; and flotation in several flotation cells arranged in series.
After the dissolving of waste-paper into a suspension of fibrous material and the separation of small particles of printing ink, or the like impurities, from the fibers, the particles of printing ink, or the like, are floated out of the suspension of fibrous material with the aid of lime soap. This is generally carried out by means of a row of flotation cells arranged in series, and which the suspension traverses. As a result of this, the particles of printing ink, or the like, float upward, together with the lime soap, and can be removed from the surface of the cells as scum. At present, flotation cells of this kind are only effective with consistencies of suspended matter of 1%.
With thicker consistencies of matter, the beneficial effect of flotation is reduced because the thickness of the greater consistency matter provides resistance to flow which can be too great for bubbles and bits of lime soap. A definite minimum period of dwell time of suspension per cell is required (approximately one minute) and a total minimum time of approximately ten minutes is also required for effective flotation. Thus, in the case of large scale waste-paper processing, one requires corresponding space and either a high number of cells or cells of large volume to achieve a desired level of whiteness of the suspension of fibrous material.